Harden the shared textbox-question component and its mobile flow parents
to prevent runtime-1 "undefined is not a function" errors seen in
CloudWatch (chunk 5615 / textbox-question.vue).
- Coerce modelValue to a string in textbox-question setup and value getter
- Ensure v-maska always receives a string mask
- Guard the value watcher when handleChange is unavailable
- Fix mobile-details v-model bindings (remove invalid this. prefix)
- Coerce address fields from store to strings before binding
- Correct malformed modelValue prop definitions on vehicle-protected-question
and service-zip-question
- Add unit test for null modelValue coercion
